A ram air turbine (RAT) is a device for generating emergency supplemental power in a wide variety of aircraft. A RAT may generate hydraulic power, electric power or both. A RAT incorporates a turbine that extracts power from an air stream proximate the aircraft in flight. The turbine is coupled to suitable power generating equipment, such as a hydraulic pump for hydraulic power and an electric generator for electric power.
The placement of a RAT on some aircraft is such that the aircraft landing gear deployment causes the landing gear to pass in front of the RAT, thereby blocking incoming airflow to the RAT. The reduction in airflow causes the RAT to stall. When the RAT stalls, it continues to rotate at a low speed, typically approximately 800 rpm, such that equilibrium exists between the turbine torque and the load torque.
Once the stall occurs, the RAT may not start back up, even when the landing gear completes its deployment and it no longer blocks the airflow to the RAT. A further reduction in the load torque is required for the RAT to re-start. The RAT then accelerates back up to full speed.
The current method of reducing load torque during initial start-up for a RAT of the hydraulic type comprises bypassing the aircraft hydraulic load until the RAT reaches full speed up. A “volume fuse” bypasses the aircraft load for this purpose. Unfortunately, this method of reducing load torque is only satisfactory during start-up, when the RAT speed is approximately zero rpm. This method does not reduce load torque for stall recovery when the RAT has stalled to a speed of around 800 rpm because the volume fuse does not reset.